Initial fabrication requirements for micromachines, such as microactuators, micropumps, and microengines, were focused on structural robustness, such as mechanical yielding and fracture strength. The development of improved silicon-based materials and techniques to evaluate their properties at the microscale have mitigated to some extent the concern of structural integrity as a critical design factor in micromachines. As a result, micromachines with satisfactory fatigue strength have been fabricated as disclosed in M. Mehregany, S. D. Senturia, and J. H. Lang, “Measurement of Wear in Polysilicon Micromotors,” IEEE Transactions on Electron Devices, Vol. 39, No. 5, 1992, pp. 1136-1143, S. F. Nagle and J. H. Lang, “A Microscale Electric Induction Machine for a Micro Gas Turbine Generator,” presented at 27th Annual Meeting of the Electrostatics Society of America, Boston, Mass., June 1999, and J. Sniegowski and E. Garcia, “Surface-Micromachined Geartrains Driven by an On-Chip Electrostatic Microengine,” IEEE Electron Device Letters, vol. 17, no. 7, 1996, p. 366.
Unfortunately, as problems with structural robustness were being addressed, other problems with the bearing surfaces in these micromachines become apparent. More specifically, the operation of moving bearing surfaces in micromachines at extremely high relative velocities resulted in high rates of wear micromachines at extremely high relative velocities resulted in high rates of wear and early seizure of the bearing surfaces. By way of example, this type of wear damage resulted in an undesirable gap between a bearing surface and a shaft as illustrated in FIG. 1 and disclosed in S. L. Miller, G. LaVigne, M. S. Rodgers, J. J. Sniegowski, J. P. Waters, and P. J. McWhorter, “Routes to Failure in Rotating MEMS Devices Experiencing Sliding Friction,” Proc. SPIE Micromachined Devices and Components III, Vol. 3224, Austin, September 1997, pp. 24-30.
To understand the cause of this wear and early seizure requires an understanding of the forces being applied to these bearing surfaces. As a load is transmitted from one bearing surface to the other, a gas or liquid film is squeezed or wedged between the deforming and moving surfaces, creating film pressure and surface shear tractions which attempt to keep the surfaces separated. The film pressure in turn induces structural deformation of the interacting surfaces. The ways in which bearing structural deformation interacts with lubricant film behavior is often referred to as elastohydrodynamic lubrication (EHL).
Bearing wear is induced between opposing surfaces through the contact behavior of surface asperities. In the absence of a gas or liquid lubricant, the asperities need to carry the entire load, and high wear rates can only be controlled through the development of surface coatings or treatments. When gas or liquids are available, load can be carried through the wedging and squeezing action of the entraining surfaces, and thus only a small percentage of the load is needed to be carried by asperity interaction. When gas or liquid films become larger than approximately three times the standard deviation of combined surface roughness, the probability of asperity interaction is very low and observed wear is practically nonexistent, regardless of surface morphology. This so-called full-film EHL condition is the optimal design target for lubricated bearing systems.
Unfortunately, this optimal design target for bearing systems with rigid surfaces can not always be achieved so wear damage and early seizures of these bearing systems continue. Efforts have been made to develop surface treatments to the rigid surfaces of these bearing systems, but these efforts have met with limited success.